Among the public cellular mobile telecommunication stems, or PLMN (Public Land Mobile Network), one of the most reputable certainly is the pan-European 900 MHz GSM system (Global System for Mobile communications), and its immediate descendant 1800 MHz DCS (Digital Cellular System). GSM is a second generation system, conforming with specifications published in the form of recommendations by specialized supranational organizations (CEPT/CCITT, within ETSI/ITU-T) whose objective is to standardize the operation of telecommunication systems proposed by the different manufacturers, in order to make them compatible and therefore able to communicate.
In the design of mobile transceiver systems, the most determining factor in design development is the choice of the access type, which is intended to be implemented on the physical channel for distribution of the available band among the different users. The access technologies, most used by the second-generation systems, are: the FDMA technology (Frequency Division Multiple Access) that performs the frequency division multiple access, the TDMA technology (Time Division Multiple Access) that performs the time division multiple access, and the SDMA technology (Space Division Multiple Access) that performs the space division multiple access.
In the FDMA technology, each user can avail of his own frequency channel shared by no other user during all the time required by the service; this case, named SCPC (Single Channel Per Carrier) is typical of the first generation analogical systems. In the TDMA technology, a single frequency channel is allocated to several users at different times called timeslots; during a timeslot only one user may transmit and/or receive on the frequency allocated to that timeslot (which can vary from one timeslot to the next one in case of Frequency Hopping). In the SDMA technique a single frequency channel is allocated to several users at the same time; discrimination between the different users is performed through the recognition of the different arrival directions of radio signals. In a same mobile system, the above mentioned access technologies can be used separately, or together in order to take avail of possible synergies. The GSM system uses a mixed FDMA-TDMA technology that, with respect to the pure FDMA, avoids an excessive use of carriers, while with respect to the pure TDMA technology, it avoids the construction of too long frames that cannot be proposed.
In the PLMN systems, the user may send information to the base station while receiving information from the latter. This communication method is called Full-duplex and may be performed using technologies both in the frequency and in the time sectors. The FDD (Frequency Division Duplexing) technology used in GSM employs different bands for the uplink section (uplink) and the downlink one (downlink). Both the bands are separated by an unused gap band to enable the appropriate radio frequency filtering. The TDD (Time Division Duplexing) technology employs different service times for the uplink and downlink sections, in respect with all the channels accessed in both the transmission directions. If the time separation between the two service times is small, transmission and reception seem simultaneous to the user.
FIG. 1 shows a summary but explanatory block diagram of the functional architecture of a mobile system of the GSM or DCS type.
The symbols MS (Mobile Station) indicate mobile telephone sets, also for cars, hereinafter called Mobiles, radio connected to the respective transceivers TRX (not visible in the figure) belonging to relevant base transceiver stations BTS (Base Transceiver Station) spread over the territory. Every transceiver TRX is connected to a group of antennas whose configuration guarantees uniform radio coverage of the cell served by the BTS. A set of N adjacent cells, which engage as a whole all the available carriers of the mobile service, is called cluster, the same carriers can be re-employed in adjacent clusters. Several BTS fixed stations are connected to a common controller of base transceiver station denoted BSC (Base Station Controller) through a physical carrier.
The set of several BTS controlled by a BSC forms a functional system denoted BSS (Base Station System).
Several BSS systems are connected to arm automatic switch for mobile, known as MSC (Mobile Switching Centre), directly or through a TRAU (Transcode and Rate Adaptor Unit) block allowing the sub-multiplexing of 16 or 8 kbit/s channels on 64 kbit/s connection lines, thus optimizing its utilization. The TRAU block performs a transcoding from the 64 kbit/s of voice to the 13 kbit/s of GSM Full Rate (or to the 6,5 kbit/s of GSM Half Rate) enabling their convey in 16 kbit/s or 8 kbit/s flows.
The MSC block is connected in turn to an automatic switch of the PSTN (Public Switched Telephone Network) and/or ISDN (Integrated Services Digital Network) land network.
Near the MSC automatic switch two data bases named HLR (Home Location Register) and VLR (Visitor Location Register) are generally installed, not visible in the figure; the first contains stable data of every Mobile MS, while the second contains the variable ones; both the bases cooperate in enabling the system to trace a user who makes widely moving on the territory, extended to the different European countries.
Furthermore, the station controller BSC is connected to a Personal Computer LMT (Local Maintenance Terminal) that allows for the man/machine dialogue to an Operation and Maintenance Center OMC performing supervision, alarm management, evaluation of traffic measurements, etc., functions named O&M (Operation & Maintenance), and finally to a SGSN [Serving GPRS (General Packet Radio Service) Support Node] block specified in GSM 04.64 for the package switching data service. The SGSN node is connected to two separate GGSN (Gateway GPRS Support Node) nodes, out of which, the first performs the interworking function with the external package switching networks, and is connected to other SGSN nodes through a GPRS dorsal network based on the IP protocol (Internet Protocol), while the second one is connected to the PSTN/ISDN automatic switch. The SGSN node is also directly connected to the PSTN/ISDN block trough an signalling-only interface (hatched line).
The figure shows vertical hatched lines that delimit the interface boundaries between the main functional blocks, more in particular, Um denotes the radio interface between MS and BTS, Abis indicates the one between BTS and BSC, and A indicates the interface between TRAU and MSC or directly between the latter and BSC. The following additional interfaces are directly indicated on the respective connections: Asub indicate the interface between BSC and TRAU, T indicate the RS232 interface between BSC and LMT, O indicates the interface between BSC and OMC, Gb indicates the interface between BSC and SGSN, Gs indicates the interface between SGSN and MSC, Gn indicates the interface between SGSN and GGSN of the dorsal GPRS, and finally Gp indicates the interface between SGSN and PSTN/ISDN. The above mentioned interfaces are described in the following GSM recommendations: 04.01 (Um), 08.51 (A-bis), 08.01 (A), 12.20 and 12.21 (O), 04.60 (Gb). The SGSN node is at the same hierarchical level of a MSC center, it follows the individual locations of mobiles and performs the safety and access control functions. To this purpose, the HLR register is enriched with information on the GPRS user.
Whichever public land mobile network (PLMN) that intends to offer users a service quality standard comparable to that offered by the fixed telephone network, shall necessarily acquire a complex signalling system. In the GSM system, a protocol with several hierarchical levels is used for the management of the telephone signalling present at the different interfaces. The protocol has been mainly obtained from the one used in the TACS analogue mobile systems and in the PSTN telephone systems, adjusting it to the new requirements of Um air interface and to those arising from the users' mobility. The levelled structure enables to subdivide the functions of the signalling protocol into overlapped block groups on the control plane (C-Plane), and to describe it as a succession of independent stages. Each level avails of the communication services made available by the lower level and offers its own to the higher level. Level 1 of the above mentioned protocol is closely connected to the type of the physical carrier used for connection at both the sides of the different interfaces; it describes the functions required to transfer the bit flows on the radio connection to the Um interface and on the land connections to Abis and A interfaces. Level 1 of the land connections is described in the recommendations CCITT G.703 and G.711. Level 2 develops functions controlling the proper flow of messages (transport functions) aimed at the development of a virtual carrier free of errors between the connected points. Level 3 , called network level, and the higher levels, develop message processing functions to control the main application processes relevant to the management of connections and to the control of calls with respect to the users' mobility.
In the GSM mobile telephone system, the mobile station MS performs a given activities even in absence of calls. Indeed, the mobile requires, as first step to be able to communicate through the network, to continuously choose a cell with which it can be associated during its movements. The above mentioned activities are included in the “Cell Selection and Reselection” function described in the GSM 03.22 and 05.08 recommendations. The mobile selects the cell with which it can be associated performing a scan of all the BCCH (Broadcast Control Channel) carriers foreseen by the GSM, known in advance by the MS because they already stored in its SIM card (Subscriber Identity Module). For everyone of them it measures the power of the received signal, demodulates the RF signal, synchronizes itself with the multiframe structure of the demodulated signal in order to be able to acquire the “System information” from the BCCH channel, among which the identifier of BSIC (Base Station Identity Code) cell and the identifier of the adjacent cells. The selected cell, called also “serving” cell, is the one that results being more reliable. The above mentioned operations are periodically performed during the state of absence of call (idle state) even when the mobile is already associated with a cell, in order to be able to associate again with a different cell if it receives a BCCH carrier of higher reliability (Cell Reselection) from the latter.
Even in dedicated mode, a procedure exists suitable to find the best resources for continuing the connection in the case of movement of the mobile from a cell to an other one. This procedure, peculiar to the cellular mobile systems, is no doubt the handover whose performance enables the network to give a control to a Mobile in order to force it to go on an other channel, of another cell in case of intra-cell handover or of the same serving cell in the case of intra-cell handover. Handover is a fundamental function for whichever mobile telephone system, since it enables the Mobile to communicate though widely moving on the territory during the communication. It prevents the deterioration of the communication channel transmission quality, which otherwise could unavoidably occur as a result of the gradual distance of the Mobile from the complex of antennas of the own serving cell. The channel jump shall furthermore be quickly performed to avoid that users may perceive a noise on the communication under way. Handover is a function mainly concerning the circuit switching systems, i.e. where connections must be maintained during the total duration of conversation otherwise information will be definitely lost during the whole duration of interruption. The package transmission on the GPRS networks does not require a handover similar to the GSM's one, i.e. with channel change during an active transmission, since packages are by their nature discontinuous and it is therefore possible to address the mobile towards mew resources taking advantages of intervals between transmission of a package and the following. As by what has just been said, it can be noticed that the invention dealt with is mainly referred to circuit switching communications, so that the problems concerning the GPRS service will be later on ignored. For a proper performance of the handover method, the mobile carries out measurements similar to those of the “Cell Selection and Reselection” procedure even during a call, and periodically updates a list of N cells, which are the most, favoured for handover.
With reference to FIG. 2, relevant to the GSM (DCS) system of FIG. 1, the level 3 sequence of messages that exists during the first phase of an external handover, i.e. with involvement of the MSC automatic switch is described. Each message starts from a network element and ends at next network element in the direction indicated by an arrow associated with the message. The network elements involved are the MS, BTS, BSC, and MSC blocks. BTS and BSC network entities are called Serving or Target depending on their reference to the present or future mobile cell. The forwards direction (towards the right side in the figure) characterizes messages in the direction from the present cell to the future cell; the backward direction characterizes the opposite direction. The steps of the time sequence of FIG. 2 are as follows:                The serving BTS station transmits to the serving BSC controller a INTERCELL_HAND_CON_IND message which includes a list of cells, among those monitored by the mobile, placed in a decreasing priority order on the basis of their capability to be selected as target to perform a handover. Depending on the manufacturing company that produces the entire BSS system, the message may be present or not, since it is always and however the BTS, or the BSC controller, (on the basis of the most appropriate choices of system) that fills in the final list on the basis of measurements of capacity of the adjacent cells performed by the mobile and forwarded with the MEASUREMENT_RESULT message. The short-term means made by the mobile are subsequently averaged out on a longer observation period, a quality parameter of the connection is therefore prepared for an n-th generic adjacent cell called “Power budget”. The Power Budgets PBGT(n) are compared with the respective thresholds, given as E&M (Operation and Maintenance) parameters, obtaining values called HO_MARGIN (n), that shall be used for Handover decisions. To this purpose, a list that includes a highest number of M preferred cells with which the Mobile could perform a Handover is filled in; from this list it is deduced that the list initially given by the mobile may even not match the final list. The above mentioned criterion is maybe one of the most known criteria adopted for Handover, but it is not the sole, for example there are criteria related to the Mobile-BTS distance or to the communication quality, just to cite only two of them.        The serving BSC while evaluating the list of cells suitable for handover selects, in the presented case of external handover, a cell not configured in the BSS area that is serving the mobile; consequently the handover procedure shall go up again to the hierarchical scale of Network Elements up to MSC. Consequently the BSS forwards the handover request to MSC through a HANDOVER_REQUIRED message including the list of possible target cells. The message in question delegates to MSC the duty of selection of the cell on which the mobile should be transferred.        At this point MSC starts searching for a cell (among those of received list) suitable to satisfy the handover request. Once the target cell has been identified, MSC will send to the targeted BSC a HANDOVER_REQUEST message on the interface A, containing different information among which the “classmarks”, i.e. a number of information describing the mobile's capacity, useful for the BSS target to select association for the new resources. These classmarks were previously supplied to MSC by the serving BSS system by means of a CLASSMARK_UPDATE message.        The BSC target sends in turn a CHANNEL_ACTIVATION message to the targeted BTS station to control the enabling of the transmitter.        The BTS target station answers back with a CHANNEL_ACTIVATION_ACK message to notify the enabling of the requested resources.        The BSC target controller sends back to MSC a HANDOVER_REQUEST_ACK message to notify that search and enabling of resources in the target cell have been successfully completed. The message conveys the control to be sent to the mobile to move it on the target cell.        MSC sends back towards the serving BSC controlled a HANDOVER_COMMAND message containing the control for transfer of the mobile to the targeted cell. This control goes through the BSC controller and the serving BTS stations until it reaches the mobile, which carries it out to complete the handover procedure.Analysis of the Technical Problem        
The imminent introduction of the third generation mobile systems (3G), or UMTS (Universal Mobile Telecommunication System), poses significant compatibility problems with the existing second-generation PLMN systems. The UMTS systems are subject to international standards published by the 3GPP consortium (3rd Generation Partnership Project). In the sector of the 3GPP standardization, two technologies of radio access CDMA (Code Division Multiple Access) have been defined; they are respectively known as, TDD UTRA (Time Division Duplex UMTS Terrestrial Transceiver access), in which the transmission directions are different in the time domain, and WCDMA UTRA (Wide band CDMA), in which the transmission directions are different in the frequency domain. The TDD UTRA technology in turn foresees two options: a first broadband option known as 3.84 Mchips TDD, and a narrow-band option, known as 1.28 Mcps TDD. The Applicant of the present invention in collaboration with the Chinese Committee CWTS (Chinese Wireless Telecommunication Standards) is actively working on the development of a standard based on the physical level of the 3GPP 1.28 Mcps TDD standard, but re-employing many of functions and procedures of the highest levels of the GSM-GPRS protocol, offering the network operators a technology able to operate on the most part of the elements of the GSM network. This standard is known as TD-SCDMA (Time Division—Synchronous CDMA), or more precisely, TD-SCDMA System for Mobile (TSM).
Because of the importance of the CDMA technique in the 3G systems, it is useful to briefly introduce this technique saying that the same utilizes reciprocally orthogonal spreading codes sequences, or whose mutual correlation may be assumed as nul. Exactly this allows for discrimination between the different users who are totalled in the transmission band, since on a channel characterized by own code sequence, the signals of other channels, as correlation result, will appear as a noise. With respect to the narrow band traditional systems, the spread spectrum technique offers the advantage of a higher insensitivity to the Rayleigh selective fading, these latter being caused by multiple reflections along the on-air path of the transmitted signal, due the fact that the spectral portion interested in the strong fading is only a very small part of the spectrum totally occupied by the active signal. The second and third generation systems may take advantage from utilization of an intelligent antenna, adding to the existing multiplexing also the SDMA one, this opportunity is already provided for in the TD-SCDMA system.
Thanks to the higher transmission speed and flexibility of resources allocation offered by the CDMA technique, the third generation systems could offer the users a number of services and applications, particularly concerning the broadband data transmission. Furthermore, the TDD serviceability offers the advantage to configure the utilization band in “spectrum gaps” corresponding to areas in which the gap required for the utilization of the FDD technology (different bands for TX and. RX separated by an unused interval) is not complied with. An other advantage of the TDD duplexing compared to the FDD one, is the possibility to withstand a data asymmetric traffic, exactly like the one originated by the IP applications, allocating more resources in downlink than in uplink. On the other hand, the 3G systems require big investments both in economical terms (infrastructures ard research), and in terms of risk assumption. As said here above, introduction of the TD-SCDMA system will offer a promising possible answer to the fact of necessity to maintain the compatibility with the preliminarily existing system GSM (DCS), allowing for a progressive and with low risk transition from the 2G systems to the 3G systems. The purpose for the near future is to preserve, where possible, the functional characteristics of GSM, while intervening on the other hand any time the impact of the new 3G technology requires necessarily ad hoc solutions. During the first phase of its development, the TD-SCDMA network may be island distributed over the existing GSM-GPRS (or DCS) network, with which it will share an important part of the network as the Core Network (in the case of circuit switching represented by MSC, HLR, VLR etc.). During this phase of integration with the existing GSM or DCS network, the roaming between both the systems will be very high. An advantageous characteristic of mobile stations (but not necessarily compulsory) is that it is a multi-system (3G, 2G). In this case a 3G mobile will be able to avail of frequency disjoined bands of the different systems to select the best target cell to which it will be connected, it could communicate with the network even in geographical areas temporarily not covered by the new 3G system, but the most important aspect is that thanks to the innovations introduced by this invention in the operation mode of the station controller, the mobile will be able to perform a so-called inter-system handover, characterized by the change over for the mobile from 3G mobile transceiver access technology to the GSM (DCS) one, the vice versa is not yet allowed for because of the choice to speed up at the maximum extent the possible the introduction of the TSM standard without requesting modifications to the GSM standardization organizations (which ignores systems other than DCS), since such a request should require a lot of time.
Ad hoc solutions for the FDMA-TDMA technology are used in the GSM system, these solutions are not directly transferable to the telephone systems in the CDMA technology, at least as regards the radio interface, which is the one that poses the most important problems. Utilization of the same core network GSM, and/or DCS, for introduction of the 3G system, advantageous in many aspects, imposes furthermore to resolve the problem of proper signalling during the delicate handover procedure. It has been observed that to avoid modifications of the core network it is necessary to allow for a handover between the different technologies only from TD-SCDMA cells to GSM. This is however not yet sufficient in the case of external handover, since MSC is directly concerned ard it is necessary to safeguard the transparency of this important component of the GSM core network towards the new technology. The handover procedure described with reference to FIG. 2 regards an external handover between cells, all GSM, and cannot obviously be provided with useful learning for performance of a handover in presence of cells of different radio access technology.
Hereinafter two operation modes concerning the management of list of cells candidate for handover between the TD-SCDMA system and the GSM/DCS system could reasonably be assumed.                A first mode, already innovative in itself, could be that of compiling homogeneous lists of cells (all the cells of each list utilize the same technology) and in selecting from time to time a list to be forwarded to the MSC.        A second mode is that consisting in compiling only one “mixed” list including cells of both the systems.        
Both the modes come from information forwarded by the mobile to the network through the MEASUREMENT_RESULT message originated by the dual-standard mobile connected in TD-SCDMA technology. This information on the status of connection (transmission power of the mobile, revel of signal perceived by the mobile on the cells adjacent to it, etc.) is crossed with the information known by the same BSS (power used in Downlink, static information relative to proximities, etc.). So, both the modes will only increase the probability of handover failure for a common reason that will appear immediately.                First mode: sending an homogeneous list of handover candidate cells to the MSC switch when this holds classmarks information (capacity of mobile in a system) that is inconsistent with the target system, and considering that by design choice the MSC switch does not have any possibility to distinguish between them the cells of both the systems, it is deduced that MSC cannot perform a conversion of the classmarks in order to make them suitable to the target system.        Second mode: considering that the start point for the commissioning of the new 3G system was to strictly avoid to bring out any hardware and software modification or adaptation to the MSC circuit switch, this important component of the GSM network “knows” only the GSM (DCS) world and ignores the existence of other access techniques (CDMA) associated with the GSM world, consequently if the above mentioned “mixed” list were forwarded to the MSC circuit switch this will not have any possibility to distinguish the cells of both the systems between them, and should consider all of them as GSM cells associated with the classmarks information stored during the set-up phase of the current call. As mentioned here above, the cause of handover failure is the same already evidenced, and i.e. the BSS system, target of the “shielded” system will interpret in the usual mode classmarks information (wrong) received by the MSC circuit switch relevant to the additional standard. In particular, if Classmarks information of the TD-SCDMA system are forwarded to the GSM (or DCS) BSS system target, it will interpret the values therein contained as if they were relative to the Classmarks Information of the GSM system, apposed at least it succeeds in decoding them.        
The technical problem highlighted for the TD-SCDMA does not obviously exist for the multi-standard system formed by the GSM 900 MHz and the DCS 1800 MHz because the MSC circuit switch has provisions to support both the standards and to properly distinguish classmarks information (GSM and DCS do not have different classmarks but only optional fields or extended fields in the semantics).
An example will better clarify the technical problem. Among classmarks information of the mobile (Classmark Information) forwarded to MSC, are the power class, the ciphering algorithm, the multi slot capacity, etc. This information relates to the operation of the mobile in a system and they are different from the operation of the same mobile in the other system. Appendix 1 includes two tables: TABLE 1 indicates the power classes of the GSM mobile, TABLE 2 the power classes of the TD-SCDMA mobile. From the comparison of codes reproduced in these two tables it appears evident how coding 2 could indicate power values very different between them on the basis of the system to which it refers, i.e.:                8 W in the case of GSM 900        0.25 W in the case of DCS 1800        1 W in the case of TD-SCDMA.        
The example relevant to the power class of the mobile is applicable to Classmark2 information having the same message structure in both the GSM (DCS) and TD-SCDMA systems, but having different semantics. There are also Classmark3 messages completely different in both the systems in terms of structure and information, therefore they are not directly comparable between them; an example peculiar to the TD-SCDMA access technology is the time switching capacity between the downlink direction and the unlink direction. Obviously nothing similar exists in the GSM since devoid of sense in FDD technique. It is useful to remember that on the basis of classmark communicated by the base station of the serving system and stored by the MSC circuit switch, this late initiates a handover procedure including classmarks information received from the serving BSS system in the HANDOVER_REQUEST message.
A second problem consequent to the new mobile scenario and to the restriction on the core network is that of the design setting of the station controller. The TD-SCDMA system does not come cut as an autonomous system but for sharing, at least at its initial phase, the GSM core network. The new 2G+3G network is therefore formed by a core network common to two different access networks. Each access network sees its own functionalities mainly concentrated in the base transceiver stations and only partially in the station controller. The station controller is in the middle between its own access network and the circuit switch MSC that belongs to the common core network. From the operational point of view, the station controller accepts at input flows of bits from each access network having the same frame organisation and conveying messages regulated by similar protocols, and furthermore it provides with standard connections at output regardless of the origin of the input information, the use of two different controllers for both the access technologies seems therefore not being cost saving.